1. Field of the Invention
The present invention relates to a method of producing a chip inductor for use in a noise filter, a transducer, or other suitable apparatus.
2. Description of the Related Art
Chip inductors are widely used as high frequency filters for eliminating radiation-noises from digital equipment, such as computers. Japanese Unexamined Utility Model Publication No. 6-50312 illustrates one known monolithic chip inductor, in which a chip element body having laminated ceramic layers is provided. A coil conductor provided between the ceramic layers is connected via through-holes provided in the ceramic layers. A coil is provided in the chip element body, and the leading and trailing portions of the coil are connected to external electrodes, respectively.
An inductor for use in a high frequency filter is required to have a large inductance and a low resistance. In general, inductance is proportional to the square of the winding number of a coil and is inversely proportional to the length of the coil. A monolithic inductor of the above-described type is difficult and costly to produce due to its complexity. Moreover, a large inductance cannot be achieved because the winding number of the coil cannot be increased. Further, the resistance is relatively large because the coil conductor is constructed as a film-shape electrode.
To solve the problems described above, a method of molding an inductor is described in Japanese Unexamined Patent Application Publication No. 8-191022, in which a magnetic ceramic is extrusion-molded into a winding-core, a conductive wire is wound around the core into a coil-shape, and another magnetic ceramic is extrusion-molded thereon to form a sheathing body. Thereafter, the ceramics are fired, and external electrodes are arranged to cover both ends of the fired magnetic core, and thereafter, bonded to both ends of the fired magnetic core. Thus, the external electrodes are connected to both ends of the coil-shaped conductive wire. In this case, the production method is less complicated than with the monolithic inductor, and less costly because a metallic wire is used for the coil-shaped conductive wire. Additionally, an inductor made by the above-described method produces the desired high inductance and a low resistance.
In the above-described production methods, both portions of the magnetic ceramic that eventually become the winding-core and the sheathing body are formed by extrusion molding. The density of a molding product produced by extrusion molding is not sufficiently high. Further, in some cases, the sheathing body contains voids in the periphery of the coil and cavities are formed between the winding-core and the sheathing body. Moreover, a binder is needed to combine the ceramic particles to each other. This binder causes the formation of pores during firing. Therefore, it has been difficult to produce high-quality inductors using this method.
Furthermore, when the sheathing body is extrusion-molded, the coil is often formed eccentrically with respect to the center portion of the sheathing. Accordingly, an inductor having stable magnetic properties cannot be produced. Further, when firing is performed on eccentric coils, warpage or cracks may develop, due to the shrinkage of the ceramic when it is fired.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method of producing a chip inductor that produces a high-quality inductor and greatly reduces warpage and cracks caused by firing-shrinkage.
According to a preferred embodiment of the present invention, a method of producing a chip inductor is provided which includes the steps of inserting a conductive wire made of a metallic wire into a metallic mold, supporting both the end portions of the conductive wire on supporting-portions provided on the inside of the metallic mold so as to position the conductive wire in the approximate center portion of the metallic mold, casting magnetic ceramic slurry into the metallic mold, molding the ceramic slurry cast in the metallic mold by wet pressing to obtain a molding body having the conductive wire embedded therein, firing the molding body, and providing external electrodes on both of the end surfaces of the fired magnetic core such that the external electrodes are connected to both the end portions of the conductive wire.
As described above, the conductive wire is inserted into the metallic mold, the magnetic slurry is cast, and thereafter, the wet pressing is carried out. With this method, both of the end-portions of the conductive wire are supported on the supporting-portions provided on the inside of the metallic mold so that the conductive wire is positioned in the approximate center portion of the metallic mold. Thereby, the conductive wire does not become eccentric during the wet pressing. The supporting portions are supporting-grooves provided on the inside of the metallic mold. The molding body having the conductive wire embedded therein is produced by the wet pressing method. The molding body produced by the wet pressing method has a tight ceramic structure and a higher density as compared with one produced by extrusion molding method. Further, since the ceramic slurry is compressed, the amount of binder required is greatly reduced, and in many instances no binder is required. Therefore, a high quality inductor is produced by firing the molding body because of the high density magnetic core produced by firing. Further, the generation of pores is greatly reduced due to the reduced amount of a binder required.
As described above, the conductive wire is inserted into the metallic mold, and the wet pressing is carried out. Thus, the inductor can be formed by one molding cycle. Therefore, the production process is greatly simplified as compared with the production process for the laminated inductor. This production process is also greatly simplified as compared with an extrusion molding method. In the case where the coil-shaped wire is used as the conductive wire, a high inductance can be obtained at a lower resistance as compared with the laminated inductor.
When the linear conductive wire is used, the inductance is low as compared with that obtained when the coil-shaped conductive wire is used, however, the direct current resistance is further reduced.
When the molding body produced by wet pressing as described above is fired, the ceramic material is shrunk via firing. In this case, although the ceramic shrinks, the conductive wire does not shrink, or shrinks substantially less than the ceramic. When the coil-shaped conductive wire is used, voids are formed inside the coil. A flux may flow into the voids from the outside, affecting the characteristics of the inductor. Further, in some cases, in addition to the formation of voids, cracks are formed within the coil due to the firing-shrinkage. Moreover, where the molding is used to obtain a plurality of the molding bodies in one cycle, such that a long coil-shaped conductive wire is used, the coil-shaped conductive wire may be deflected if only the end-portions of the coil-shaped conductive wire are supported on the supporting portions of the metallic mold as described above. If the molding is used in this state, the coil-shaped conductive wire may not be disposed precisely in the center of the core along its entire length.
Thus, according to another preferred embodiment of the present invention, the winding-core made of the fired magnetic ceramic is inserted into the coil-shaped conductive wire, before the coil-shaped conductive wire is inserted into the metallic mold. Since the winding-core arranged inside the coil-shaped conductive wire is not shrunk, no voids are formed inside the coil-shaped conductive wire by firing, and moreover, formation of cracks, caused by firing-shrinkage, is prevented. Further, the winding-core is inserted into the coil. Accordingly, even if the coil length increases, deflection of the coil is prevented by the winding-core. Thus, a high quality inductor is obtained as compared to conventional methods and inductors.
The winding-core may have the same or a different composition from that of the magnetic core provided outside the coil. If the winding-core has the same composition as the magnetic core provided outside the coil, a magnetic core which is homogeneous inside and outside its coil is obtained. If the compositions are different, the magnetic permeabilities inside and outside the coil can be varied. Thus, the characteristics of an inductor can be easily altered.
Other features, elements, characteristics and advantages of the present invention will become apparent from the following description of the present invention and the accompanying drawings.